ACS and pPDH are two enzymes that are responsible for the generation of acetyl CoA in the plastids, e.g., chloroplasts, of plants. ACS generates acetyl CoA as follows:acetate+ATP+CoASH→acetyl-CoA+AMP+PPi,wherein ATP represents adenine triphosphate, CoASH represents coenzyme A, acetyl-CoA represents acetyl coenzyme A, AMP represents adenine monophosphate, and PPi represents inorganic pyrophosphate, and wherein the acetate includes that which results from the conversion of acetaldehyde and NAD+ to acetate and NADH, wherein the acetaldehyde, in turn, results from the breakdown of pyruvate, which releases CO2. pPDH generates acetyl CoA as follows:pyruvate+CoASH+NAD+→acetyl-CoA+CO2+NADH,wherein NAD+ represents nicotinamide adenine dinucleotide and NADH represents the reduced form of NAD+ and wherein the pyruvate results from glycolysis. Glycolysis involves the conversion of sugar phosphates, which have been produced from starch, photosynthesis or the importation of triose and hexose phosphates from the cytosol, to pyruvate.
Various studies of relative activity of enzymes in embryos and leaves of plants, such as spinach, castor bean, barley and Brassica have been conducted (see, Kang and Rawsthorne, Plant J. 6: 795-805 (1994); Miernyk and Dennis, J. Exper. Bot. 34: 712-718 (1983); Smith et al., Plant Physiol. 98: 1233-1238 (1992); Liedvogel and Bauerle, Planta 169: 481-489 (1986); Murphy and Leech, FEBS Letter 77: 164-168 (1977); Roughan et al., Biochem. J. 158: 593-601 (1976); Roughan et al., Biochem. J. 184: 565-569 (1978); Roughan et al., Biochem. J. 184: 193-202 (1979); Springer and Heise, Planta 177: 417-421 (1989); Schulze-Siebert and Shultz, Plant Physiol. 84: 1233-1237 (1987); and Heintze et al., Plant Physiol. 93: 1121-1127 (1990)). Such studies suggest that acetate is the preferred substrate for fatty acid synthesis in chloroplasts, while pyruvate is the preferred substrate for fatty acid synthesis in plastids in embryos.
The acetyl CoA so produced is then involved in fatty acid biosynthesis, i.e., the synthesis of the basic building blocks of membrane lipids, fats and waxes. A similar reaction is effected by mtPDH in the mitochondrion.
ACS is exclusively found in the plastids of plants and is strongly regulated by light (Sauer and Heise, Z. Naturforsch 38c: 399-404 (1983)). The amount of ACS is fairly constant between spinach, pea and amaranthus chloroplasts; there is about 20% more in corn chloroplasts. Given that the partially purified enzyme is completely DTT-dependent suggests that its activity in vivo may be regulated by the ferredoxin/thioredoxin system (Zeiher and Randall, Plant Physiol. 96: 382-389 (1991)). There is some potential for weak feedback inhibition by acetyl CoA. The enzyme also has a high pH requirement, along with a dependency on a high ATP(Mg2+-ATP)/ADP ratio (Sauer and Heise (1983), supra). The ACS reaction should be substrate saturated because Km values for acetate are between 0.02 and 0.10 mM in spinach (Sauer and Heise (1983), supra; Zeiher and Randall (1991), supra; and Treede and Heise, Z. Naturforsch 40c: 496-502 (1985)), peas (Treede and Heise (1985), supra), amaranthus (Roughan and Ohlrogge, Anal. Biochem. 216: 77-82 (1994)) and potatoes (Huang and Stumpf, Arch Biochem. Biophys. 140: 158-173 (1970)), whereas the concentration of cellular acetate is estimated to be about 1.0 mM (Kuhn et al., Arch Biochem. Biophys. 209: 441-450 (1981)).
The pPDH appears to have the same general structure as mtPDH, being composed of a pyruvate dehydrogenase component (E1α and E1β), a transacetylase component (E2), and dehydrolipoamide dehydrogenase (E3) subunits. The molecular weight of pPDH and its cofactor requirements are also similar to mtPDH, although affinities for NAD+ and TPP vary somewhat (Camp and Randall, Plant Physiol. 77: 571-577 (1985); Miernyk et al. (1983), supra; and Conner et al., Planta 200: 195-202 (1996)). pPDH, which is less sensitive to acetyl CoA than mtPDH, has an optimal pH of about 8.0 and requires about 10 mM Mg2+ for maximal activity. While the activity of mtPDH is controlled by a sophisticated kinase/phosphatase system, which phosphorylates and thereby inactivates the E1α subunit, pPDH is not subject to such regulation. However, pPDH is strongly regulated by the NADH/NAD+ ratio and is moderately regulated by light. Regulation by ATP, NADPH, fatty acyl CoAs and glycolytic intermediates is minor (Camp et al., Biochim. Biophys. Acta 933: 269-275 (1988); and Qi et al., J. Exp. Bot. 47: 1889-1896 (1996)).
PDH activity varies from one tissue to the next with mtPDH activity varying 15-fold and pPDH activity varying 6-fold (Lernmark and Gardestrom, Plant Physiol. 106: 1633-1638 (1994)). The ratio of pPDH/mtPDH also varies between plants, with 6.5 times more activity in the chloroplasts than in the mitochondria of wheat leaves to 6.7 times more activity in the mitochondria than in the chloroplasts of peas. Although chloroplasts have proportionally less PDH activity than mitochondria in pea as compared to wheat, the chloroplasts have nearly as much PDH as mitochondria in absolute terms.
ACL is an enzyme that is responsible for the generation of acetyl CoA. ACL generates acetyl CoA as follows:citrate+ATP+CoASH→acetyl-CoA+oxaloacetate+ADP+Pi,wherein ADP represents adenosine diphosphate and Pi represents orthophosphate and wherein the citrate is that which is generated in the TCA cycle in the mitochondrion. The activity of ACL has been found to correlate with lipid accumulation in developing seeds of Brassica napus L. (Ratledge et al., Lipids 32(1): 7-12 (1997)) and in the supernatant of a developing soybean (Glycine max L. Merr., var. Harosoy 63) cotyledon homogenate (Nelson et al., Plant Physiol. 55: 69-72 (1975)). ACL also has been found in crude extracts from the endosperm tissue of germinating castor bean (Ricinus communis cv. Hale) and has been found to be maximally active in 4-5-day old seedlings (Fritsch et al., Plant Physiol. 63: 687-691 (1979)).
PDC is a cytosolic enzyme that is responsible for the generation of acetaldehyde from pyruvate. PDC generates acetaldehyde from pyruvate as follows:pyruvate→acetaldehyde+CO2.The acetaldehyde so produced can be acted upon by ALDH.
ALDH is responsible for the generation of acetate from acetaldehyde. ALDH generates acetate from acetaldehyde as follows:acetaldehyde+NAD++H2O→acetate+NADH++H+.The acetate so produced can then enter the plastids, where it can be converted to acetyl CoA through the action of ACS.
ACH is an enzyme that is known to exist in yeast and is believed to exist in the mitochondria of plants. ACH is believed to generate acetate from acetyl CoA pools present in the mitochondria. The acetate so produced is then believed to be released from the mitochondrion into the cytosol. The cytosolic acetate can then enter the plastids, wherein it can be converted to acetyl CoA through the action of ACS.
Acetyl CoA is the common precursor of a large number of phytochemicals, which have widely varied biological functions and which represent renewable, energy-rich products of agriculture (e.g., fats, oils, waxes, isoprenoids and bioplastics (e.g., polyhydroxybutyrate) or which affect agricultural production (e.g., flavonoids, stilbenoids, isoprenoids and malonyl derivatives) (Goodwin and Mercer, Introduction to Plant Biochemistry, 2nd ed., Pergamon Press, New York (1988)). These phytochemicals are synthesized either by the carboxylation or sequential condensation of acetyl CoA.
Carboxylation of acetyl CoA (via the intermediate malonyl CoA) leads to the biosynthesis of fatty acids (e.g., membranes, oils, cuticle, suberin and cutin), flavonoids (e.g., pigments, phytoalexins and plant protection), stilbenoids (e.g., plant protection and pharmaceuticals), acridones, malonic acid, and a variety of malonyl derivatives (aminocyclopropane carboxylic acids, D-amino acids, flavonoids and pesticides). Fatty acids are the building blocks of all cellular membranes. In addition, fatty acids are utilized in developmentally regulated processes in the biogenesis of seed oils, cuticle, cutin and suberin. Most seed oils are triacylglycerols. Flavonoids are a group of water-soluble phenolic compounds that have a wide range of biological activities as pigments and they accumulate in responses of plants to biotic and abiotic stresses (e.g., drought, fungal and bacterial pathogens, and salt stress). Stilbenoids are thought to play a role in plant defense mechanisms. The acridones are a class of alkaloids that have a wide spectrum of antimicrobial, antimolluscosidal and antiviral activities. Numerous malonyl derivatives exist in plants, including those of D-amino acids, flavonoids and xenobiotics, such as pesticides. The malonation of aminocyclopropanecarboxylic acid, which is the precursor of ethylene, may influence the generation of the hormone ethylene.
The condensation of acetyl CoA (via the intermediates acetoacetyl CoA and HMG CoA) leads to the biosynthesis of isoprenoids. Examples of isoprenoids include sterols, phytoalexins, abscisic acid, gibberellins, phytoene, β-carotene, phytol, natural rubber, plant protection and pharmaceuticals. Isoprenoids are also significant constituents of many essential oils and fragrances.
In addition, and of great excitement to the biotechnology industry, acetoacetyl CoA is the precursor for the production of a potentially new agricultural product from transgenic plants, namely polyhydroxybutyrate (PHB, a type of bioplastic). Research to date indicates that the production of transgenic bioplastics may be limited by the supply of acetyl CoA (Nawrath et al., PNAS USA 91: 12760-12764 (1994)).
The pathways that utilize acetyl CoA as a precursor are spatially and temporally compartmentalized. Fatty acids and sterols are synthesized by all cells for membrane biogenesis. The accumulation of most of the other acetyl CoA-derived phytochemicals is highly cell-specific and occurs in specific subcellular compartments at particular stages of development or in response to particular environmental signals. For example, acetyl CoA is required in plastids for de novo fatty acid synthesis, which produces 18-carbon fatty acids. The elongation of 18-carbon fatty acids to fatty acids of 20 carbons and longer requires a cytosolic acetyl CoA pool. Acetyl CoA is also required in the cytosol for the biosynthesis of isoprenoids, flavonoids, and several, if not all, of the malonated derivatives. In addition, fatty acid synthesis in the plastid should be maximal during triacylglycerol deposition in oil seed as well as during times of maximum membrane formation, such as during the conversion of proplastids to chloroplasts.
Therefore, in view of the above, there remains a need for materials and methods to alter the level of enzymes involved in acetyl CoA production and, consequently, acetyl CoA levels in plants. Accordingly, it is an object of the present invention to provide such materials and methods. These and other objects and advantages of the present invention, as well as additional inventive features, will become apparent to one of ordinary skill in the art from the following description.